Malaria caused by Plasmodium falciparum claims the lives of three million children per year, mainly in Africa. A vaccine is not available and chemoprophylaxis alone is unlikely to significantly reduce transmission. Aside from the threat of insecticide resistance, existing vector control strategies that focus on indoor use of insecticides may not reach all of the disease-transmitting population or may induce a behavioral shift, given underlying population heterogeneities in vector resting and biting behavior-- even in vectors considered to be primarily "endophilic" and "endophagic" (indoor resting and biting) such as Anopheles gambiae. In Africa, A. gambiae is the most important vector. This mosquito has adapted rapidly to climatically diverse and anthropogenic environments. Instrumental to its adaptive flexibility are polymorphic chromosomal inversions. Of particular relevance are alternative arrangements on the left arm of chromosome 2 (2La or +a) that are preferentially associated with contrasting environments (arid and humid) through entirely unknown physiological and/or behavioral mechanisms. Based on frequency distribution maps of 2La, this arrangement reaches 100% in arid savannas while in humid rainforests only the alternative arrangement (2L+a) is found. Thus without the benefit of 2La, A. gambiae would be limited to rainforest areas where this mosquito is not necessarily the most abundant or even the best malaria vector. At a local level, 2La reaches its highest frequency during the dry season and in samples captured resting indoors at night where the nocturnal saturation deficit is higher. Thus 2La influences a key epidemiological trait-- the probability of vector-human contact-- as well as the likelihood of vector exposure to insecticide-treated walls and bed nets, through its effect on indoor biting and resting behavior. The ultimate goal of this project is to identify the genes and gene networks in 2La that confer resistance to aridity-- a phenotype or suite of phenotypes that leads to increased vector-human contact and malaria transmission at both local and geographic scales. To achieve this goal, we propose a multidisciplinary and integrative approach that combines phenotypic and molecular analysis to begin to tease apart the functional genomics of 2La through three specific aims: (1) Identify phenotypic traits associated with alternative arrangements of 2La;(2) Identify sequence differences between arrangements that may contribute to phenotypic differences;(3) Associate genotypic and phenotypic differences by comparing patterns of gene expression. The short-term outcome of this program will be linkages between adaptive phenotypes and underlying candidate genes, leading to specific hypotheses about how 2La confers resistance to aridity and impacts the probability of vector-human contact. The longer term benefits are two-fold. The first is improved implementation, evaluation and design of vector control, based on a mechanistic understanding of what we now call "adaptive flexibility". In other words, gaining a detailed understanding of genetic, physiological and behavioral attributes of 2La that are linked to aridity tolerance and indoor resting behavior will significantly improve our ability to predict the epidemiological impact of existing and novel vector control strategies, and can lead to the design of more comprehensive strategies resistant to evasion by components of the vector population. The second benefit is that a successful outcome in the study of functional genomics of aridity resistance in 2La will serve as a general model for studying the functional genomics of many other medically important traits in A. gambiae. PUBLIC HEALTH RELEVANCE Existing control strategies are inadequate and threatened due to development of resistance. Novel vector control strategies are needed, especially for the primary African malaria vector Anopheles gambiae. In this mosquito, genic variations carried on rearranged chromosomes (known as chromosomal inversions) confer traits that are essential to vector success in contrasting (arid or humid) environments. As such, they impact vector distribution seasonally and spatially, including indoor resting and biting behavior. Therefore, they influence a key epidemiological trait-- the probability of vector-human contact-- as well as the likelihood of vector exposure to insecticide-treated walls and bed nets. Unfortunately, the relevant genetic, physiological and behavioral mechanisms underlying these heterogeneities in the vector population are completely unknown. Detailed understanding of these traits will significantly improve our ability to predict the epidemiological impact of existing and novel vector control strategies, and can lead to the design of more comprehensive strategies resistant to evasion by components of the vector population.